Food Sachet or Container

ABSTRACT

A sachet or food container is formed of two plastics sheets joined to one another around the edges of an internal pocket of the sachet or container by welded seals. The compositions of the two plastics sheets differ one from the other and are selected such that the strength of the seals is controllable by varying at least one welding parameter selected from temperature, pressure and dwell time. The two sheets are welded to one another using a first set of welding parameters to produce strong seals in some of the seam regions bounding the internal pocket, while other seam regions bounding the pocket are welded using a second set of welding parameters to produce weak seals at which the sachet will burst open preferentially when the pocket is pressurised.

FIELD OF THE INVENTION

The present invention relates to the manufacture of sachets for use in a beverage machine and of packets and containers for enabling food to be heated in a microwave oven.

BACKGROUND OF THE INVENTION

Beverage machines are known which inject a liquid, usually hot water, under pressure into a sachet having a sealed pocket containing an infusible product, such as ground coffee or tea leaves, or soluble solids such as hot chocolate powder or granules. The sachet bursts under the pressure of the injection water and a beverage is dispensed into a cup. One example of such a pressure rupturable sachet is to be found in GB 2392374.

It is desirable to design sachets of this kind such that they will always burst in the same place and dispense the beverage in a controlled manner. This is in order to avoid soiling the machine itself, the outside of the dispensing cup or the machine operator.

The same requirement for a controlled release of the pressure within a container is encountered when food is sold in a package that can be used in a microwave oven. In this case, it is important for the container to open before the pressure rises excessively. Furthermore, if a small opening can be formed to enable controlled release of steam from within the container, it can act as a small pressure cooker. While the invention will be described below with particular reference to beverage machine sachets, it should be appreciated that it applies equally to any food container that is sealed by a sheet of plastics material that has a seam around its periphery.

Sachets have hitherto been made by welding two identical sheets of a suitable plastics material to one another along all the seam lines where a strong seal is required and by gluing the two sheets to one another, by means of a suitable adhesive, in the regions of the seams where it is desired to produce a weak seal to form a discharge opening in the sachet for the beverage to be discharged. It will be appreciated that the terms “strong” and “weak” refer here to the ease with which the two sheets will separate when placed under pressure. Furthermore these terms are relative rather than absolute. It is only required that the weak seals should rupture before the strong seals because, once they have ruptured, they will vent the pressure in the pocket and reduce the possibility of the sheets also separating at the strong seals.

It is thus desirable to provide a container having regions with a weak seal which are produced without the use of an adhesive. By avoiding the use of an adhesive, it is possible to save cost while enabling the sachet to open more consistently and predictably. One can also never be sure of the long term effects of any adhesive that finds its way into the dispensed beverage which has to flow over it when the seal ruptures and the sachet is opened.

U.S. Pat. No. 4,759,472 describes a pressure rupturable sealed sachet which relies on seals having different strengths, weak seals being formed by welding at a lower temperature than strong seals. The latter reference is however silent with regard to the composition of the two sheets that are welded to one another.

The plastics sheet materials used to make sachets usually take the form of laminates i.e. where two or more layers are bonded together to provide the required tensile strength, barrier properties and heat sealability. Typically PET (polyethylene terephthalate also commonly referred to as polyester) could be used as the outer layer to provide strength, and the inner heat seal layer would be of polypropylene. Barrier properties can be provided by an intermediate layer of aluminium foil, or the PET may be metallised, or treated by some other means. Such a sachet material will henceforth describe as a PET/PP laminate.

Experiments using such plastics sheet materials did not prove successful. In particular, polypropylene was not found suitable as it did not offer sufficient control over the strength of the weld and the results obtained were inconsistent.

Other more complex materials were tried in which polypropylene was combined with other polymers that inherently give softer seals, and have a much wider sealing window—e.g. polyethylene co-polymers. These materials were found to provide the facility to vary seal strength over a much wider range than standard polypropylene and they do find application as easy peel films, as used in peel off lids for containers. Because of the primary uses of such copolymers, they will herein be referred to as easy peel materials. The experiments in which easy peel materials were welded to each other were only partially successful, because it was found difficult to achieve consistent strong seals in the required locations.

OBJECT OF THE PRESENT INVENTION

The present invention seeks therefore to optimise the selection of the composition of the plastics sheets from which a sachet is made, to allow both strong and weak seals to be achieved reliably where required by controlling the welding parameters used in forming the seals.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a sachet or food container formed of two plastics sheets joined to one another around the edges of an internal pocket of the sachet or container by welded seals, wherein the compositions the two plastics sheets differ one from the other and are selected such that the strength of the seals is controllable by varying at least one welding parameter selected from temperature, pressure and dwell time, and wherein the two sheets are welded to one another using a first set of welding parameters to produce strong seals in some of the seam regions bounding the internal pocket, while other seam regions bounding the pocket are welded using a second set of welding parameters to produce weak seals at which the sachet will burst open preferentially when the pocket is pressurised.

The term “sheet” is being used herein to include the body of a container such as a cup which is not flat but has a rim around its mouth to which the second sheet is welded to form a sealed enclosure or pocket.

The invention is predicated on the discovery by the inventors that best results are obtained by welding to each other plastics sheets which have a different composition from one another. Thus while welding polypropylene to itself and easy peel materials to themselves did not produce satisfactory result, good results were obtained by welding a polypropylene layer to an easy peel layer.

For example, using an RDM lab sealer, it was found that strong seals were obtained with welds formed at 180° C., 2 bar pressure for 5 seconds. On a Lloyd tensile tester operating at 300 mm/min, the seal strength was around 2.2 kgf/25 mm. A weak seal, around 0.75 kgf/25 mm, was formed by lowering the temperature to 135° C. at the same dwell and pressure.

In this particular case, temperature was the most useful welding parameter, but it will be appreciated that on continuous sealing packing machines, the other two welding parameters could be important. It should also be appreciated that the same strategy could also be applied where other forms of energy (e.g. ultrasound) are used to weld plastic sheets.

In the preferred embodiment of the invention, one of the plastics sheets comprises a laminate of polyester and polypropylene and the other is an easy peel plastics material, the easy peel plastics material being preferably a copolymer of polypropylene and polyethylene.

An example of easy peel material which has been found to be particularly effective is currently sold by Global Sealing Technologies under the trade mark Propeel.

In addition to modifying the welding parameters, the weak seals are advantageously formed by narrower welds than the strong seals.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described further, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a section through a sachet, and

FIG. 2 is a plan view of the sachet in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The sachet 10 shown in the drawings is essentially the same as that disclosed in WO2004/019741 to which reference may be made for a more detailed explanation of the sachet and the manner in which it is used in a beverage dispensing machine. The sachet 10 is formed of two plastics sheets 12, 14 welded to one another along seams 22 to define a sealed pocket 20 filled with a granular beverage material 18, such as ground coffee, tea leaves, or chocolate or soup powder.

A spacer 16 is provided to keep the two sheets 12, 14 apart and hot water injected into the spacer 16 passes into the pocket 20 through radial passages in the spacer 16 and contacts the powder 18. The spacer 16 is designed to prevent the brewed beverage from contacting the needle that injects hot water into the sachet to prevent contamination of the needle. As the pocket 20 fills with hot water, the pressure in the pocket rises until a point is reached when a weak seal region 24 along the seam 22 ruptures and creates an opening for the brewed beverage to flow into a cup.

The present invention is concerned with the manner in which the weak seal region 24 is produced. In the prior art, while the remainder of the seam 22 was produced by welding, the two plastics sheets 12, 14 were commonly glued to each other in the region 24 to form a weak seal. By contrast, in the present invention, the entire seam 22 surrounding the pocket 20 is welded but different welding conditions are used to create a weaker seal in the region 24 than in the remainder of the seam 22.

Conventional plastics sheet materials used in making sachets comprise a laminate having an outer layer of polyester (PET) and an inner layer of polypropylene (PP). Though PET is strong and resistant to tearing, unlike PP, it cannot, on account of its narrow melting point temperature band easily be welded to itself. The use of a laminate combines the advantages offered by both materials. However, when regular PET over PP laminates are welded to one another, they produce only strong seals. If a temperature and pressure are applied for a given time then either the sheets will not be welded to one another at all or else the resultant weld will not peel apart easily.

The present invention is based on the discovery that combinations of different materials offer more control over the strength of a seal by varying the welding parameters. In particular, it has been found that when welding Propeel, a material intended only to make easily peeled seals, to a conventional PET over PP laminate one can selectively obtain a strong seal or a weak seal by varying the welding temperature and dwell and, to a lesser extent, the applied pressure.

The bag shown in FIG. 2 is therefore made of two sheets of dissimilar material, as described above, which have a seam 22 that exhibits strong seal qualities around most of the perimeter of the pocket 20. Because of the strong seal, the two sheets will not separate from one another under the action of the water that is introduced into the pocket 20 under pressure. By contrast, a small region 24 at the bottom of the sachet is welded at a lower temperature and/or with a short dwell time to form a weak seal allowing the two sheets 12, 14 to peel apart when the pressure in the pocket is sufficiently high.

It will also be noticed from FIG. 2 that the weak seal region 24 of the seam 22 is narrower than the remainder of the seam. This assists in the peeling apart of the two sheets 12, 14 when the pocket 20 is under pressure and can be configured to leave a small area beneath the seal region where the two sides 12, 14 are never bonded to each other. This area, if present, can act as a funnel to constrain the liquid to flow downwards out of the sachet into the dispensing cup and prevent it from splashing sideways. One may also leave a similar small area above the seal region, which will assist in rupturing the seal.

The regions of the seam 22 with different seal strength may all be formed at the same time by the use of a die with accurate temperature control but it is probably simpler to form two separate welds sequentially under different welding conditions as this allows the temperature and dwell time to be set differently for the two welds.

It will be appreciated that the above described method of forming strong and weak seals can also be applied to a container in the form of a rigid cup or bowl that has a sheet of plastics material welded to the rim surrounding its mouth to seal its contents. If such a container is used to heat food in a microwave, the weal seal will permit the pressure in the container to be released safely and automatically, avoiding the need to pierce the container before it is heated. This is advantageous because it allows hot food to be sold using an automated vending machine in that a container can be transported from a storage station directly to an oven without first having to pierce it. 

1. A sachet or food container formed of two plastics sheets joined to one another around the edges of an internal pocket of the sachet or container by welded seals, wherein the compositions of the two plastics sheets differ one from the other and are selected such that the strength of the seals is controllable by varying at least one welding parameter selected from temperature, pressure and dwell time, and wherein the two sheets are welded to one another using a first set of welding parameters to produce strong seals in some of the seam regions bounding the internal pocket, while other seam regions bounding the pocket are welded using a second set of welding parameters to produce weak seals at which the sachet will burst open preferentially when the pocket is pressurised.
 2. A sachet or food container as claimed in claim 1, wherein one of the plastics sheets comprises a laminate of polyester and polypropylene and the other is an easy peel plastics material.
 3. A sachet or food container as claimed in claim 2, wherein the easy peel plastics material is a copolymer of polypropylene and polyethylene.
 4. (canceled)
 5. A sachet or food container as claimed in claim 1, wherein the weak seals are formed by narrower welds than the strong seals.
 6. A sachet or food container as claimed in claim 3, wherein the weak seals are formed by narrower welds than the strong seals. 